1. Field of Invention
The present invention relates to a decoder which decodes original signals from supplementary information indicating the relationship between the original signals and a downmix signal obtained by downmixing the original signals, and in particular relates to a technique for decoding original signals with high accuracy in the case where supplementary information indicates the phase difference and the gain ratio of the original signals.
2. Description of the Related Art
Recently, a technique known as Spatial Codec (spatial coding) has been developed. This technique aims at compressing and coding realistic sounds from multiple channels using a very small amount of information. For example, the AAC format, which is a multi-channel codec widely used as an audio format for digital television, requires a bit rate of 512 kbps or 384 kbps per 5.1 channels. However, Spatial Codec aims at compressing and coding multi-channel signals using a very small bit rate of 128 kbps, 64 kbps or 48 kbps.
As a technique to realize this, Patent Reference 1, for example, discloses that it is possible to compress and code realistic sounds using a small amount of information by coding the phase difference and the gain ratio of channels.
On the other hand, some compression schemes which have been widely used partially employ such a technique of coding the phase difference and the gain ratio of channels. For example, the above-mentioned AAC format (ISO/IEC 13818-7) employs a technique known as Intensity Stereo.    Patent Reference 1: U.S. Patent Publication No. UP2003/0236583A1.